1. Field of the Invention
The present invention relates to a friction plate for a wet-type multi-plate clutch used as a brake or a clutch of an automatic transmission of a vehicle and the like.
2. Related Background Art
In a friction plate having good properties in which drag torque is reduced and the clinging in friction engagement at an initial stage during the friction engagement is prevented by providing an oil passage or an oil groove in a friction material of the friction plate, a technique in which an oil passage extending from an inner peripheral side to an outer peripheral side and a groove having a closed end and gradually narrowing from the inner peripheral side to the outer peripheral side are provided has been proposed, for example, as disclosed in Japanese Patent Application Laid-open No. 2005-76759.
In the Japanese Patent Application Laid-open No. 2005-76759, excessive lubricating oil which generates viscous resistance during an idle rotation is discharged via the oil passage, and a gap or clearance between friction surfaces is maintained properly by the groove having the closed end and gradually narrowing from the inner peripheral side to the outer peripheral side, thereby reducing idle rotation drag and initial clinging torque.
Although such a friction plate is excellent in the points that the drag torque during the idle rotation is reduced and that the initial clinging of the friction engagement is eliminated, in such a friction plate, there is variation in an oil discharging ability from the friction surfaces during the engagement depending upon the difference or variation in the porosity of the friction material so that there is variation in coefficient of friction of each friction plate at the initial engagement.
Oil pressure for urging the friction plates of the automatic transmission is tuned in order to achieve smooth speed change; however, if there is great variation in the oil discharging ability from the friction surfaces of the respective friction plates, it is difficult to adjust individually, which may cause transmission shock.